He spoke after the US government said it was rethinking its opposition to arming rebels fighting Bashar al-Assad's regime. American defence secretary Chuck Hagel said arming the Syrian opposition was "always an option".

Dozens of people, including women and children, were killed on Friday by regime forces in Bayda, a village near the Mediterranean coast, according to the UK-based Syrian Observatory for Human Rights.

The debate over how to bring an end to the Syrian conflict has intensified over recent days as Britain has found "very persuasive" evidence of the use of chemical weapons in the conflict. But concerns remain over Islamist elements among the opposition and the increasingly sectarian nature of the conflict.

Mr Hammond said the Government may decide to take stronger action after the European arms embargo expires at the end of this month. He told BBC2's Newsnight: "The Americans have said today that they are rethinking whether to supply weapons to the opposition. We have never closed off that option.

"We currently have a European arms embargo which expires at the end of May, we haven't yet decided what approach we are going to take to any renewal or modification of that embargo. But we want to keep all our options open as we work out the best way to deal with this despicable regime which is slaughtering its people by the thousands."

The Defence Secretary said that the Government would need "incontrovertible" evidence of the use of chemical weapons if they were to be part of an international response. He said "ideally" that such a response should be taken through the United Nations Security Council.

"We've certainly got some very persuasive evidence (of the use of chemical weapons)," he said. "But if we set ourselves the test of whether this evidence would be persuasive in a UK or a US court, we still don't have evidence of that quality. Given the experience we have had both in the UK and the US in relation to the Iraq war and the evidence that was presented to support our intervention there, I think it is very clear that our publics would expect us to leave no stone unturned in establishing that the evidence is compelling and conclusive before we take any further action."

A No 10 spokeswoman said: "We continue to seek changes to the EU arms embargo so that we have the flexibility to do more in the future if necessary. Those discussions are continuing in Brussels. As the PM has said, we think those changes are necessary because there is currently stalemate and our current approach is not working.

"We need to find ways to put more pressure on the regime so that we can advance a political transition in the country but important to be clear that we have not taken any decision to arm the opposition."

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